I tried doing a system restore but when I clicked System Restore in System Tools, nothing happened. I did the same for a second time and a blank window opened.
I closed that window, tried to start it again and nothing happened. I tried to start it once more and that blank window opened again. So the first time you try to start it, nothing happens and if you do it again a blank window comes up.
I had a look at System Restore in services.msc and it says "Started". I stopped and then re-started it but nothing changed. So in this case I think it looks like formatting and re-installing Windows. I scanned my hard drives with Eset Smart Security and no threats were found. But I know from experience that E.S.S. can't detect all malicious software. Perhaps this is the work of some viruses running in background. Anyway thanx a lot for your help.